1. Field of the Invention
This invention relates to a non intrusive method aid system for the measurement of a distance, or the variations over time thereof, constituting the thickness or the variations thereof of a compressible dielectric material located between one or several other closely spaced solid dielectric layers of constant thickness and a conductive or semi-conductive surface, and more particularly for detecting and measuring the vibrations of the stator bars of an electrical generator or motor while said generator or motor is operating and whether said generator is a hydrogenerator or a turbogenerator.
2. Description of Related Art
The stator winding insulation system of an electric generator is exposed to harsh operating conditions. When the wedging system no longer holds the stator bars in place, vibrations set in. As stator bar retention within the slot gradually weakens, partial discharges slowly increase in intensity, and insulation is undermined. As this action progresses, side-erosion destroys the semi-conductive painting of the bar and intensifies the vibrations of the bar and the partial discharges. If such harmful vibration activity is not detected from the beginning and corrective actions are not taken, rapid insulation deterioration may lead to irreversible damages or aging so severe that repair and even partial or complete rewinding become necessary.
Several detection systems and techniques address this problem.
A method commonly employed for assessing the stator wedge tightness of a generator consists in hitting with a hammer each of the stator wedges to detect from the sound produced how tightly the wedges are held in place by the pressure of the springs attached to them. Such method has the disadvantage of requiring the generator to be stopped in most cases and the rotor to be dismantled at least partly.
More recently, U.S. Pat. No. 5,493,894 (Dailey et al.) presents a method and a system assembly by which an impact is created against the stator core lamination and a vibration sensor receives vibrations from said stator coil lamination. Such method and system require the electrical generator to be stopped and opened and the rotor completely dismantled for the temporary installation of the impact assembly and of the vibration sensor. Such method presents the disadvantages of being static, intrusive and cannot describe in a continuous manner how the wedge behaves while the generator is running.
U.S. Pat. No. 5,524,474 (Lavallee et al.) describes a method and assembly which uses a thin flat piston-mounted sensor inserted in the air-gap between the rotor and stator and positioned in front of the stator wedge to be evaluated. A variable pressure is applied to the piston of the sensor, causing a displacement of the wedge which is used to determine its tightness. This method and apparatus does not necessitate the dismantling of the rotor, but such measurement is static, while the generator is stopped, and cannot describe in a continuous manner bow the wedge behaves while the generator is running.
U.S. Pat. No. 4,675,670 (Lalonde et al.) describes a capacitive method to measure dynamically, the air-gap and its variation over time existing between the rotor and the stator of an electrical generator while the generator is running. Such method has been adapted by the licensee of said patent. VibroSystM Inc., also an assignee of the present application, to measure the vibrations of stator wedges of an electric generator by embedding capacitive sensors in stator slots, each such capacitive sensor facing a stator bar and measuring the gap between itself and the stator bar. This method, while efficient, presents the drawback of requiring the replacement of the existing wedge with a special wedge which accommodates the embedded capacitive sensor and its supporting assembly. This method is intrusive and locally modifies the existing stator configuration and installation.
Therefore, there is a need for a non intrusive and dynamic method to detect on a continuous basis the changes in tightness of the wedges of stator bars in electric generators and motors, while said generators and motors are running.